1. Technical Field
Embodiments of the subject matter disclosed herein generally relate to methods and systems and, more particularly, to mechanisms and techniques for towing seismic sources underwater.
2. Discussion of the Background
Marine seismic data acquisition and processing generate a profile (image) of a geophysical structure under the seafloor. While this profile does not provide an accurate location of oil and gas reservoirs, it suggests, to those trained in the field, the presence or absence of these reservoirs. Thus, providing a high-resolution image of the geophysical structures under the seafloor is an ongoing process.
Reflection seismology is a method of geophysical exploration to determine the properties of earth's subsurface, which are especially helpful in the oil and gas industry. Marine reflection seismology is based on using a controlled source of energy that sends the energy into the earth. By measuring the time it takes for the reflections to come back to plural receivers, it is possible to evaluate the depth of features causing such reflections. These features may be associated with subterranean hydrocarbon reservoirs.
A traditional system for generating the seismic waves and recording their reflections off the geological structures present in the subsurface is illustrated in FIG. 1. A vessel 10 tows an array of seismic receivers 11 provided on streamers 12. The streamers may be disposed horizontally, i.e., lying at a constant depth relative to a surface 14 of the ocean. The streamers may be disposed to have other than horizontal spatial arrangements. The vessel 10 also tows a seismic source array 16 that is configured to generate a seismic wave 18. The seismic wave 18 propagates downward toward the seafloor 20 and penetrates the seafloor until eventually a reflecting structure 22 (reflector) reflects the seismic wave. The reflected seismic wave 24 propagates upward until it is detected by the receiver 11 on the streamer 12. Based on the data collected by the receiver 11, an image of the subsurface is generated by further analyses of the collected data.
The seismic source array 16 may include plural individual source elements. The individual source elements may be grouped in sub-arrays, so that each source array may include one or more sub-arrays. The individual source elements may be distributed in various patterns, e.g., circular, linear, at various depths in the water. FIG. 2 shows a vessel 40 towing two cables 42 provided at respective ends with deflectors 44. Plural lead-in cables 46 are connected to streamers 50. The plural lead-in cables 46 also connect to the vessel 40. The streamers 50 are maintained at desired separations from each other by separation ropes 48. Plural individual source elements 52 are also connected to the vessel 40 through lead-in umbilical 54 and to the lead-in cables 46 through gun tag ropes 56. This configuration does not allow accurate control of the plural individual source elements and also involves a large number of cables for towing each source, which increases the existing drag.
A limitation of the existing structures is the high number of seismic sources and, implicitly, a high number of source vessels necessary for performing a wide-azimuth (WAZ) survey. WAZ seismic surveys provide better illumination than traditional marine surveys in complex geologic areas by essentially “shining the light” on the formations from many directions. The “light” comprises seismic signals over a wide range of azimuths, or angles of incidence, on the subsurface reflectors. WAZ surveys typically use multiple source and streamer vessels to obtain this advantage.
Thus, it is desirable to design a new towing system that uses fewer vessels for achieving the WAZ survey or uses the same number of vessels but increases the number of sources towed by the existing vessels.
Another limitation of the existing structures is today felt for arctic acquisitions. In this harsh environment, pieces of ice float at the surface of the water and are likely to interfere with the floats that maintain the sources at desired depths. Thus, there is a need to have a source system that does not have a float component to interact with the floating ice.